Consultation of the prior art shows that this synthesis of alkanesulphonyl chlorides from mercaptans is carried out, under batchwise or continuous conditions, by bringing chlorine and the mercaptan into contact in a concentrated aqueous hydrochloric acid solution (Patents FR 1 598 279, DE 1 811 768, U.S. Pat. No. 3,626,004, JP 52-020970, U.S. Pat. Nos. 3,600,136 and U.S. Pat. No. 3,993,692) or in pure water (Patents DE 2 845 918, JP 52-008283, U.S. Pat. No. 3,248,423 and U.S. Pat. No. 4,280,966), the objective being to obtain an alkanesulphonyl chloride with a good yield and the best possible purity, that is to say while avoiding the formation of troublesome by-product or intermediate compounds. One of the most troublesome among these compounds is 1-chloroalkanesulphonyl chloride RCHCl—SO2Cl; alkanesulphinic acid RCH2—SO2H, alkyl alkanethiosulphonate RCH2—SO2—SCH2R and alkanesulphinyl chloride RCH2—SOCl, optionally in combination with their α-chlorinated derivatives, are also encountered. These compounds, for the most part of low stability, are the source of the colouring of alkylsulphonyl chloride over time.
In order to avoid the formation of such compounds, it is advantageous to operate with an excess of water. This makes it possible to greatly restrict the formation of 1-chloroalkanesulphonyl chloride and alkanesulphinic acid, alkyl alkanethiosulphonate or alkanesulphinyl chloride is no longer encountered in alkanesulphonyl chloride prepared in this way.
The hydrochloric acid produced by the reaction remains predominantly trapped in the water, which has to be introduced in large amounts in order to ensure good selectivity for alkanesulphonyl chloride. This is one of the reasons why the abovementioned Patents FR 1 598 279, DE 1 811 768, U.S. Pat. No. 3,626,004, JP 52-020970, U.S. Pat. No. 3,600,136 and U.S. Pat. No. 3,993,692 recommend introducing the water in the form of a concentrated aqueous hydrochloric acid solution, so as to recover a pure hydrochloric acid gas, the impurities being predominantly trapped in the acidic aqueous phase.
However, as explained above, the formation of the α-chlorinated derivative RCHCl—SO2Cl, a troublesome impurity for the applications of the chloride RCH2—SO2Cl which is very difficult to separate by standard purification techniques, is thus promoted.
On analysing the prior art, it emerges that the processes provided do not take into account the quality of the hydrochloric acid coproduced and that there was no interest in its enhancement in value. Recovered after separation by settling of the reaction mixture, it accumulated various by-products, such as sulphuric acid, sulphinic and sulphonic acid, a small amount of alkanesulphonyl chloride, and the like, and it is therefore unsuitable in this state for marketing.
In summary:
1) when the water is introduced in the form of a concentrated aqueous hydrochloric acid solution, a gaseous HCl of good quality is recovered but the alkanesulphonyl chloride obtained comprises relatively large amounts of the α-chlorinated derivative RCHCl—SO2Cl;
2) when the water is introduced pure, the alkanesulphonyl chloride is of very good quality (very little 1-chloroalkanesulphonyl chloride) but the aqueous phase recovered is contaminated by sulphuric acid and organic compounds and the concentration of the hydrochloric acid obtained does not necessarily correspond to a commercial specification.